An arrhythmia is an abnormal heartbeat pattern. One example of arrhythmia is bradycardia wherein the heart beats at an abnormally slow rate or wherein significant pauses occur between consecutive beats. Another example is a tachycardia wherein the heart beats at an abnormally fast rate. With atrial tachycardia, the atria of the heartbeat abnormally fast. With ventricular tachycardia, the ventricles of the heart beat abnormally fast. Though often unpleasant for the patient, a tachycardia is typically not fatal. However, some tachycardias, particularly ventricular tachycardia, can trigger ventricular fibrillation wherein the heart beats chaotically, such that there is little or no net flow of blood from the heart to the brain and other organs. Ventricular fibrillation, if not terminated, is fatal. Hence, it is highly desirable to prevent or terminate arrhythmias, particularly arrhythmias of the type that may lead to ventricular fibrillation.
One technique for preventing or terminating arrhythmias is overdrive pacing wherein the implantable cardiac stimulation device applies electrical pacing pulses to the heart at a rate somewhat faster than the natural or “intrinsic” heart rate of the patient. Overdrive pacing for prevention of tachyarrhythmias is also taught in U.S. Pat. No. 6,058,328 by Levine et al. and in U.S. patent application Ser. No. 09/471,788 by Florio et al. For bradycardia, the cardiac stimulation device may be programmed to artificially pace the heart at a rate of 60 to 80 pulses per minute (ppm) to thereby prevent the heart from beating too slow and to eliminate any long pauses between heartbeats. To prevent a tachycardia from occurring, the cardiac stimulation device artificially paces the heart at a rate of at least five to ten beats per minute faster than the intrinsic heart rate of the patient. In other words, a slight artificial tachycardia is induced and maintained in an effort to prevent an actual tachycardia from arising. If an actual tachycardia occurs, such as a supraventricular tachycardia (SVT) wherein the heart may begin beating suddenly at 150 beats per minute (bpm) or more, the cardiac stimulation device senses the tachycardia and immediately begins pacing at a rate of at least five to ten ppm faster than the tachycardia, then slowly decreases the pacing rate in an attempt to slowly reduce the heart rate back to a normal rate thereby terminating the tachycardia.
It is believed that overdrive pacing is effective for at least some patients for preventing or terminating the onset of tachycardia for the following reasons. A normal, healthy heart beats only in response to electrical pulses generated from a portion of the heart referred to as the sinus node. The sinus node pulses are conducted to the various atria and ventricles of the heart via certain, normal conduction pathways. In some patients, however, additional portions of the heart also generate electrical pulses referred to as “ectopic” pulses. Each pulse, whether a sinus node pulse or an ectopic pulse has a refractory period subsequent thereto during which time the heart tissue is not responsive to any electrical pulses. A combination of sinus pulses and ectopic pulses can result in a dispersion of the refractory periods, which, in turn, can trigger a tachycardia. By overdrive pacing the heart at a uniform rate, the likelihood of the occurrence of ectopic pulses is reduced and the refractory periods within the heart tissue are thereby rendered more uniform and periodic. Thus, the dispersion of refractory periods is reduced and tachycardias are substantially avoided. If a tachycardia nevertheless occurs, overdrive pacing at a rate faster than a tachycardia helps to eliminate any ectopic pulses that may be occurring and thereby helps terminate the tachycardia.
Thus it is desirable within patients prone to tachycardia to ensure that most beats of the heart are paced beats, as any unpaced beats may be ectopic beats. A high percentage of paced beats can be achieved simply by establishing a high overdrive pacing rate. However, a high overdrive pacing rate has disadvantages as well. A high overdrive pacing rate may be unpleasant to the patient, particularly if the artificially-induced heart rate is relatively high in comparison with the heart rate that would otherwise normally occur. A high overdrive pacing rate may also cause possible damage to the heart or may trigger more serious arrhythmias, such as a ventricular fibrillation. A high overdrive rate may be especially problematic in patients suffering from heart failure, particularly if the heart failure is due to an impaired diastolic function. Indeed, a high overdrive rate may actually exacerbate heart failure in these patients. Also, a high overdrive rate may present a problem in patients with coronary artery disease because increasing the heart rate decreases diastolic time and decreases perfusion, thus intensifying ischemia. Also, the need to apply overdrive pacing pulses operates to deplete the power supply of the stimulation device, perhaps necessitating surgical replacement of the stimulation device.
Accordingly, it is desirable to provide a degree of overdrive pacing that is high enough to eliminate tachycardias, but not so high as to pose other risks. For many patients, the optimal degree of overdrive pacing as measured by the percentage of paced beats out of total heartbeats is between 85% and 95%. In other words, out of every twenty heartbeats, only two or three beats should be intrinsic beats, the rest should be paced beats. The degree of overdrive pacing is affected by a number of programmable control parameters, which the physician adjusts using an external programming device in an attempt to achieve the desired degree of overdrive pacing. Among the parameters that affect the degree of overdrive pacing are 1) the overdrive pacing response function or response “slope”; 2) the number of overdrive events; 3) the recovery rate; 4) the base rate; 5) the rest rate; and 6) the circadian base rate. Briefly, the overdrive pacing response function specifies an overdrive pacing rate to be applied when overdrive pacing is triggered with the overdrive pacing rate dependent upon the heart rate in effect when overdrive pacing is triggered. Overdrive pacing is triggered, for example, upon the detection of two intrinsic heartbeats out of the last sixteen paced or intrinsic beats. The number of overdrive events specifies the number of consecutive beats to be paced following triggering of overdrive pacing. The number of overdrive events may be a function of heart rate in effect when overdrive pacing is triggered, or may be one value, which applies to all rates. The recovery rate specifies a rate decrement by which the pacing rate is to be decreased after the number of overdrive events, have been paced. The base rate specifies a standard non-overdrive pacing rate for use while the patient is awake. The rest rate specifies an alternative non-overdrive base rate for use while the patient is asleep or otherwise at profound rest. The circadian base rate is yet an alternative base rate which maybe used instead of the standard base rate and rest rate. The circadian base rate is typically set equal to the average active heart rate while the patient is awake and set equal to the average rest heart rate while the patient is at rest.
With regard to the overdrive pacing response functions, typically one or more overdrive pacing response function is pre-programmed into the stimulation device. Each specifies an overdrive pacing rate for each corresponding intrinsic heart rate throughout a broad range of detectable heart rates, such as from 55 bpm to 150 bpm. FIG. 1 illustrates an exemplary set of three response functions or slopes each of which specifies an overdrive pacing rate (shown on the y-axis) as a function of the intrinsic atrial rate (shown on the x-axis). The intrinsic atrial is rate is shown in bold. Briefly, the physician programming the stimulation device selects one of the response functions. Thereafter, the stimulation device detects the intrinsic heart rate then determines the overdrive pacing rate corresponding to the intrinsic rate by examining the selected response function then paces the heart at that rate. If response function #1 has been selected and the intrinsic rate is 70 bpm, an overdrive pacing rate of 75 ppm is specified by the response function and the heart is overdrive paced at that rate. If response function #2 has been selected and the intrinsic rate is 70 bpm, an overdrive pacing rate of 80 ppm and the heart is overdrive paced at that rate. The higher the overdrive rate as compared to the intrinsic rate, the more aggressive the overdrive pacing. By providing multiple response functions, the physician can thereby set the aggressiveness of overdrive pacing. (Typically, a half dozen or more response functions are provided. For clarity in illustrating the response functions, only three are shown in the figure.) Note however that the predetermined response functions are generally linear, i.e. for most response functions the differences between the overdrive pacing rate and the current heart rate is fairly uniform throughout the entire range of detectable heart rates. Considering response function #1, regardless of whether the current heart rate is 55 bpm or 150 bpm, the overdrive rate is 5 ppm higher. For some response functions, such as response function #3, the overdrive pacing increase over the heart rate is somewhat less for high heart rates than for low heart rates, but the difference from high to low changes fairly gradually and uniformly over the full range of heart rates. As a result of this general “linearity” of the response functions, the selection of a different response function usually results in a fairly uniform increase or decrease in all overdrive pacing rates for all ranges of heart rates.
In use, the stimulation device monitors the heart of the patient and, if a predetermined intrinsic rate is detected, overdrive pacing is automatically triggered. The overdrive pacing rate is determined using the selected response function and the heart rate at the time overdrive is triggered. The stimulation device then overdrive paces the heart at the selected overdrive pacing rate for the programmed number of overdrive events. Thereafter, the stimulation device slowly decreases the overdrive pacing rate by the rate decrement specified by the programmed recovery rate until additional intrinsic beats are detected, then the device repeats the process to determine a new overdrive pacing rate and paces accordingly. If the heart rate is increasing quickly, such as may occur with an episode of tachycardia, the stimulation device may still detect intrinsic beats even while overdrive pacing is being applied. If so, the stimulation device immediately determines a new higher overdrive pacing rate based on the selected response function and the new heart rate. Again, if intrinsic beats are still detected, the overdrive pacing rate is increased per the response function. In this manner, the overdrive pacing rate may quickly be increased to 150 ppm or more in response to a tachycardia such as SVT.
Ultimately, the overdrive rate will be increased to the point where it exceeds the intrinsic rate of the tachycardia and hence no intrinsic beats will be detected. The pacing rate is eventually decreased using the recovery rate until two intrinsic beats out of sixteen cycles are again detected and the pacing rate is increased again. Assuming that overdrive pacing has succeeded in terminating the tachycardia, the recovery rate will ensure that the pacing rate decreases slowly back down to a normal rate of perhaps 60 to 80 bpm. If a base rate is programmed, such as 60 bpm, the heart will be paced at the base rate even if the recovery rate would otherwise cause the rate to decrease even further. Likewise, if an alternative base rate, such as the rest rate or circadian base rate are programmed, the pacing rate will not fall below those rates either.
With this technique, so long as the intrinsic rate remains above the currently programmed base rate then, regardless of whether the intrinsic heart rate remains stable or increases or decreases rapidly, the heart is overdrive paced so that the actual heart rate of the patient remains above the intrinsic heart rate most of the time, with only occasional intrinsic heartbeats. When employing this technique, the resulting degree of overdrive pacing is a complicated function of the various programmable control parameters and of the characteristics of the heart of the patient, such as whether the patient is prone to tachyarrhythmia or bradycardia. Generally, the more aggressive the response function, the higher the degree of overdrive pacing. The more aggressive the recovery rate, the lower the degree of overdrive pacing. The greater the number of overdrive events in each sequence of overdrive pacing, the greater the degree of overdrive pacing. The higher the base rate, rest rate or circadian base rate, the higher the degree of overdrive pacing. If the patient is prone to frequent tachycardias, the degree of overdrive pacing is typically lower than otherwise. If the patient is prone to frequent bradycardia, the degree of overdrive pacing is typically higher than otherwise.
The various parameters affecting the degree of overdrive pacing are programmed by the physician in an attempt to ensure that the optimal degree of overdrive pacing is achieved of typically 85% to 95%. Unfortunately, it is quite difficult for a physician to initially determine the parameters needed to achieve the desired degree of overdrive pacing within a particular patient. Instead, the physician typically sets the various control parameters of the stimulation device of the patient to default values and then programs the device to record the resulting degree of overdrive pacing as a function of heart rate. The patient is sent home and, weeks or months later, the patient returns to the physician for a follow-up session to permit the physician to review the recorded information and to determine whether the default parameters achieved the desired degree of overdrive pacing. If the degree of overdrive pacing is too low, perhaps only 50%, the physician typically increases the number of overdrive events or selects a more aggressive response function. If the degree of overdrive pacing is too high, perhaps 100%, the physician decreases the number of overdrive events or selects a less aggressive response function. The patient is again sent home and, weeks or months later, the patient again returns to the physician so that the physician can again review the recorded degree of overdrive pacing and, if needed, re-set the number of overdrive events or the response function. This process is usually repeated several times over a period of many months until a number of overdrive events and a response function are identified that comes closest to achieving the desired degree of overdrive pacing. During this process the physician may also adjust any of the other parameters as well, such as the base rate, recovery rate etc.
As can be appreciated, the need for frequent follow-up sessions can be a considerable inconvenience to the patient and can increase overall health care costs. Also, during the potentially lengthy period of time before the control parameters are optimized, the degree of overdrive pacing is either too low or too high posing the aforementioned risks.
Accordingly, it would be highly desirable to provide an improved overdrive pacing technique permitting the desired degree of overdrive pacing to be more expediently achieved without the need for frequent follow-up sessions between patient and physician and it is to this end that aspects of the invention are directed.
Moreover, because the stimulation device has only a limited set of preprogrammed response functions, the best response function may not achieve the desired degree of overdrive pacing. A significant problem is this regard is that the predetermined response functions are substantially linear and hence the selection of a different response function usually results in only a uniform increase or decrease in all overdrive pacing rates for all ranges of heart rates. Hence, at best, the physician may only be able to achieve the desired degree of overdrive pacing for certain ranges of heart rates, such as low heart rates or high heart rates, but not for all heart rates. As a result, the best response function may achieve an overall degree of overdrive pacing of, for example, only 75%. Thus, with conventional overdrive pacing techniques, the desired degree of overdrive pacing sometimes simply cannot be achieved despite repeated follow-up sessions with the physician and despite modifying any or all of the programmable parameters.
Accordingly, it also would be highly desirable to provide an improved technique permitting the desired degree of overdrive pacing to be more precisely achieved and in particular for overcoming the disadvantages associated with fixed, predetermined response functions, and it is to this end that other aspects of the invention are directed.